Zircon ages and differentiation trend of Ordovician granitoids from the southeastern Ötztal Nappe (Texelgruppe, South Tyrol, Italy): ridge subduction at the margin of Gondwana?

The Austroalpine Ötztal Nappe and the adjacent Marlengo Slice comprise pre-Variscan basement of Neoproterozoic to Early Paleozoic metasedimentary rocks intruded by Ordovician granitoids now present as orthogneisses. LA-ICPMS U-Pb zircon dating complemented by whole-rock geochemistry on orthogneisses from the Texelgruppe, the southeastern part of the Ötztal Nappe, and the Marlengo Slice yields an Ordovician differentiation trend from ~470 Ma to ~447 Ma, evolving from arc to collision and finally post-orogenic granitoid signature. We interpret this as an expression of a contemporaneous mid ocean ridge subduction under the active margin of Northeastern Gondwana. Abundant zircon cores exhibit an age distribution inherited from the sedimentary melt source of the orthogneisses with S-type geochemistry and record influence by the Northeast-Africa-Arabia Zircon Province and the Cadomian magmatic arc.